Early antibiotic treatment can delay the development of CF chronic pulmonary P. aeruginosa infection. Recent data indicate that paranasal sinuses are an initial site of the pulmonary infection and serve as a reservoir for subsequent reinfection. The main objective of this randomized, multicenter study is to compare the efficacy of two types of antibiotic treatment: a classic eradication protocol based on antibiotics targeted to bronchial airways only, versus a new protocol including, in addition to these, the treatment of paranasal sinuses by means of nasal lavage with colistin. The CF patients will be randomized to receive either the classic or the experimental treatment, which will last 4 weeks in both groups. Eradication success will be defined as three negative, successive P. aeruginosa cultures within 6 months. The new protocol could clarify the role of paranasal sinuses in CF pulmonary infection,and prove to be a simple and useful method for further delaying the reappearance of P. aeruginosa in the lungs of CF patients.